Since alloys capable of storing and releasing hydrogen (which are abbreviated as "hydrogen storage alloys" hereinafter) were discovered and known to be applicable to the storage of hydrogen, it has been attempted to extend their application to a heat pump and a cell without limiting their use to the means of storing hydrogen. In particular, the alkaline rechargeable batteries using such alloys as negative electrode have reached nearly practical levels, and the alloys used therein have undergone successive improvements.
More specifically, the LaNi.sub.5 alloy which was first examined (Japanese Tokkai Sho 51-13934, wherein the term "Tokkai" means an "unexamined patent application") has an advantage in that it can store a large quantity of hydrogen. However, it has also disadvantages in that not only La metal is expensive but also the alloy tends to pulverize through the alternation of storage and release of hydrogen and to be corroded by an alkaline or acidic solution.
When the above-cited alloy was used as the electrode of an alkaline rechargeable battery, therefore, it had high electric capacity in the early stage of use, but the electric capacity thereof was reduced to one half or less by about 50 times' repetition of a charge-discharge cycle. That is, such a battery cannot withstand the use of long duration.
The drawbacks as described above were mitigated by partly replacing the lanthanum (La) with another rare earth element such asCe, Pr, Nd, etc., and/or the nickel (Ni) with another metal such as Co, Al, Mn, etc. (as disclosed in Japanese Tokkai Sho 53-4918, Japanese Tokkai Sho 54-64014, Japanese Tokkai Sho 60-250558, Japanese Tokkai Sho 61-91862 and Japanese Tokkai Sho 61-233969).
Those alloys, although they were somewhat inferior to the LaNi.sub.5 alloy in quantity of hydrogen stored therein, underwent improvements in liability to corrosion by alkaline and acid solutions and in charge-discharge cycle life of an alkaline rechargeable battery.
However, they were still short in charge-discharge cycle life from an industrial point of view, and so it was hard to say that they had sufficient practical utility.
The hydrogen storage alloys having the crystal structure of CaCu.sub.5 type (which are hereinafter called the AB.sub.5 type alloys) have a multi-phase texture composed of various intermetallic compounds, metals and so on. This is a main reason why such alloys are, as described above, liable to be corroded by alkaline and acidic solutions. Further, it is considered that microcracks formed in the corroded intermetallic compound part push the alloys to be finely pulverized. Of the intermetallic compounds which constitute the AB.sub.5 type alloy, it is supposed that the greater the proportion of the metal on the side A in an intermetallic compound, the more serious the corrosion by alkaline and acidic solutions.
In order to solve the above-described problems, we have made intensive studies. As a result thereof, it has been found that the corrosion resistivity and the charge-discharge cycle life of a hydrogen storage alloy can be improved when an alloy contains only the intermetallic compound named AB.sub.5 phase while every other intermetallic compound phase is absent in the alloy, thereby achieving the present invention.